#36827: Adding hash index support for exclusion constraint in PostgreSQL
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Reporter: haki | Type: New feature
Status: new | Component: contrib.postgres
Version: 6.0 | Severity: Normal
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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In PostgreSQL you can’t define a unique constraint using a hash index.
However, you can enforce uniqueness using an exclusion constraint and a
hash index. This is useful for large values that only use equality and
that are not referenced by FKs (fields like UIDs, hashes, checksums, URLs,
other large texts etc.).
However, the current
implementation(https://github.com/django/django/blob/4426b1a72dc289643e2ae8c190b8dc4b3a39daf7/django/contrib/postgres/constraints.py#L38)
of `ExclusionConstraint` in `django.contrib.postgres.constraints` is
limited to gist and spgist. It seems like the implementation haven’t
changed much since it was added in 2019.
I made a local change to allow hash to the list of allowed `index_type`s
and generated this constraint:
{{{
from django.contrib.postgres.constraints import ExclusionConstraint
class ShortUrl(models.Model):
class Meta:
constraints = (
ExclusionConstraint(
index_type='hash', # <--- this index type is currently
unsupported
expressions=[
(F('url'), '='),
],
name='%(app_label)s_url_unique_hash',
),
)
}}}
It produced the expected SQL:
{{{
ALTER TABLE "shorturl_shorturl" ADD CONSTRAINT "shorturl_url_unique_hash"
EXCLUDE USING hash ("url" WITH =);
}}}
After applying the migration the constraint worked as expected.
--
Ticket URL: <https://code.djangoproject.com/ticket/36827>
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